Home / Notes / Biology / Inheritance
Biology · Unit 10

Inheritance: MDCAT Notes and MCQs

Mendel (1866, garden pea — Pisum sativum)

  • Law of segregation: two alleles of a gene separate during gamete formation; each gamete gets one. Monohybrid F2: phenotype 3:1, genotype 1:2:1.
  • Law of independent assortment: alleles of different genes (on different chromosomes) assort independently. Example: round-yellow (RRYY) × wrinkled-green (rryy) → F1 RrYy (round yellow) → F2 9:3:3:1 (9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green).
  • Test cross: cross with homozygous recessive. RrYy × rryy → 1:1:1:1.
  • A dihybrid RrYy forms 4 types of gametes; generally 2n gamete types for n heterozygous gene pairs.

Linkage & crossing over

  • Linkage: genes on the same chromosome tend to be inherited together (T.H. Morgan, Drosophila). Linked genes do not assort independently → mostly parental combinations.
  • Crossing over: exchange of segments between non-sister chromatids of homologous chromosomes during prophase I (pachytene) at chiasmata → produces recombinants.
  • Recombination frequency (%) = recombinants / total × 100. 1% = 1 map unit (centimorgan). The farther apart two genes, the more crossing over.

Sex-linkage

  • Genes on the X chromosome = X-linked. Males (XY) are hemizygous — one recessive allele shows its effect.
  • X-linked recessive traits: haemophilia, red-green colour blindness, Duchenne muscular dystrophy. More common in males; passed from carrier mother to sons; affected father passes allele to all daughters but no sons (criss-cross inheritance).
  • Y-linked (holandric): hypertrichosis of ears — father to all sons.

Haemophilia

  • Blood fails to clot normally. Haemophilia A — lack of factor VIII (≈ 80%); Haemophilia B (Christmas disease) — lack of factor IX. Known as the "royal disease" (Queen Victoria was a carrier).
  • Carrier mother (XHXh) × normal father (XHY) → daughters: 50% normal, 50% carriers; sons: 50% normal, 50% haemophiliac.
  • Haemophiliac father (XhY) × normal mother → all daughters carriers, all sons normal.
PMDC learning outcomes for this unit
  • Mendel's laws of Inheritance: 10.1 Associate inheritance with the laws of Mendel.
  • Mendel's laws of Inheritance: 10.2 Explain the law of independent assortment, using a suitable example.
  • Gene linkage and crossing over: 10.3 Describe the terms gene linkage and crossing over
  • Gene linkage and crossing over: 10.4 Explain how gene linkage counters independent assortment and crossing-over modifies the progeny
  • X-linked Recessive inheritance: 10.5 Describe the concept of sex-linkage.
  • X-linked Recessive inheritance: 10.6 Briefly describe inheritance of sex-linked traits
  • X-linked Recessive inheritance: 10.7 Analyze the inheritance of hemophilia.

Inheritance MCQs with answers

Practise and track your score →
Q1
The F2 phenotypic ratio of a dihybrid cross is:
A1:1:1:1
B3:1
C9:3:3:1
D1:2:1
Show answer
Answer: C (9:3:3:1). Independent assortment of two gene pairs gives 9:3:3:1 in F2.
Q2
The test-cross ratio of a dihybrid (RrYy × rryy) is:
A3:1
B9:3:3:1
C1:1:1:1
D1:2:1
Show answer
Answer: C (1:1:1:1). The heterozygote makes 4 gamete types equally; the recessive parent makes only ry.
Q3
How many types of gametes can be formed by genotype AaBbCc?
A6
B8
C9
D3
Show answer
Answer: B (8). 2^n where n = 3 heterozygous pairs → 8 gametes.
Q4
Linked genes:
AAre on different chromosomes
BTend to be inherited together
CAssort independently
DAlways give 9:3:3:1
Show answer
Answer: B (Tend to be inherited together). Genes on the same chromosome stay together unless separated by crossing over.
Q5
Crossing over takes place between:
ACentromeres
BNon-sister chromatids of homologous chromosomes
CNon-homologous chromosomes
DSister chromatids
Show answer
Answer: B (Non-sister chromatids of homologous chromosomes). It occurs between non-sister chromatids during prophase I of meiosis.
Q6
Crossing over occurs during:
AInterphase
BMetaphase I
CAnaphase II
DProphase I (pachytene)
Show answer
Answer: D (Prophase I (pachytene)). Chiasmata form and exchange occurs in pachytene of prophase I.
Q7
Haemophilia A is due to deficiency of clotting factor:
AXII
BVIII
CVII
DIX
Show answer
Answer: B (VIII). Haemophilia A — factor VIII; haemophilia B (Christmas disease) — factor IX.
Q8
A carrier woman marries a normal man. The chance that their son will be haemophiliac is:
A0%
B100%
C50%
D25%
Show answer
Answer: C (50%). Sons receive either X^H or X^h from the mother: 50% affected.
Q9
A haemophiliac man marries a normal (non-carrier) woman. Their daughters will be:
AAll normal non-carriers
BAll haemophiliac
CAll carriers
D50% carriers
Show answer
Answer: C (All carriers). Every daughter receives the father's X^h and mother's X^H → all carriers.
Q10
Sex-linked recessive disorders are more common in males because males are:
AHemizygous for X-linked genes
BHomozygous
CPolyploid
DHeterozygous
Show answer
Answer: A (Hemizygous for X-linked genes). Males have one X; a single recessive allele is expressed.
← Support & MovementCirculation →